Functions srand48(), seed48(), and lcong48() are initialization entry points, one of which
should be invoked before either drand48(), lrand48(), or mrand48() is called. (Although it
is not recommended practice, constant default initializer values will be supplied automatically
if drand48(), lrand48(), or mrand48() is called without a prior call to
an initialization entry point.) Functions erand48( ),nrand48( ), and jrand48() do not require
an initialization entry point to be called first.

All the routines work by generating a sequence of 48-bit integer values,
Xi , according to the linear congruential formula

Xn+1= (aX n+c)mod m n>=0.

The parameter m = 248; hence 48-bit integer arithmetic is performed. Unless
lcong48() has been invoked, the multiplier value aand the addend value care
given by

a = 5DEECE66D16 = 2736731631558

c = B16 = 138

The value returned by any of the functions drand48(), erand48(), lrand48(), nrand48(),
mrand48(), or jrand48() is computed by first generating the next 48-bit Xi
in the sequence. Then the appropriate number of bits, according to the type
of data item to be returned, are copied from the high-order (leftmost)
bits of Xi and transformed into the returned value.

The functions drand48(), lrand48(), and mrand48() store the last 48-bit Xi generated
in an internal buffer. Xi must be initialized prior to being invoked.
The functions erand48(), nrand48(), and jrand48() require the calling program to provide
storage for the successive Xi values in the array specified as an
argument when the functions are invoked. These routines do not have to
be initialized; the calling program must place the desired initial value of Xi
into the array and pass it as an argument. By using different
arguments, functions erand48(), nrand48(), and jrand48() allow separate modules of a large
program to generate several independent streams of pseudo-random numbers, that is, the
sequence of numbers in each stream will not depend upon how many
times the routines have been called to generate numbers for the other
streams.

The initializer function srand48() sets the high-order 32 bits of Xi to
the 32 bits contained in its argument. The low-order 16 bits of
Xi are set to the arbitrary value 330E16 .

The initializer function seed48() sets the value of Xi to the 48-bit
value specified in the argument array. In addition, the previous value of
Xi is copied into a 48-bit internal buffer, used only by seed48(), and
a pointer to this buffer is the value returned by seed48(). This
returned pointer, which can just be ignored if not needed, is useful
if a program is to be restarted from a given point at
some future time — use the pointer to get at and store
the last Xi value, and then use this value to reinitialize using seed48()
when the program is restarted.

The initialization function lcong48() allows the user to specify the initial Xi
the multiplier value a, and the addend value c. Argument array elements
param[0-2] specify Xi, param[3-5] specify the multiplier a, and param[6] specifies the 16-bit
addend c. After lcong48() has been called, a subsequent call to either
srand48() or seed48() will restore the ``standard'' multiplier and addend values, a
and c, specified above.

Usage

Programmers should use /dev/urandom or /dev/random for most random-number generation, especially for
cryptographic purposes. See random(7D).